$ D = \left[\begin{array}{rrrr}-1 & 1 & 3 & 1 \\ -1 & 0 & 0 & -1 \\ 3 & -1 & 1 & -1\end{array}\right]$ What are the dimensions of matrix $ D$ ?
Solution: The first dimension is the number of rows in the matrix. How many rows does $ D$ have? The second dimension is the number of columns in the matrix. How many columns does $ D$ have? So, $ D$ is a 3 $\times$ 4 matrix.